SUMMARY
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When a brush is assigned to "My favourites" the category still appears in the "Assign to tag" menu. This can be confusing, because it may prompt one to reassign a brush to my favourites.
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STEPS TO REPRODUCE
1. Open the brush preset docker 
2. Right click the brush and assign a random brush to the "My favourites" category
3. Right click the brush again and go to the "assign to tag" menu once again. 
4. Notice the category "My favourites" still appears



OBSERVED RESULT
4. Notice the category "My favourites" still appears

EXPECTED RESULT
Just like any other category, the category should disappear from the "Assign to tag" menu if the brush is already assigned to this tag. 

SOFTWARE/OS VERSIONS
Windows: 10
Qt Version: 5.12.12